In the discussion of the state of the art that follows, reference is made to certain structures and/or methods. However, the following references should not be construed as an admission that these structures and/or methods constitute prior art. Applicant expressly reserves the right to demonstrate that such structures and/or methods do not qualify as prior art against the present invention.
Carbon nanotubes can be synthesized by techniques that include: arc discharge between graphite electrode, chemical vapor deposition (CVD) through catalytic decomposition of hydrocarbon and laser evaporation of a carbon target. Examples of these methods are described in the literature: U.S. Pat. Nos. 4,572,813; 4,663,230; 5,165,909; 5,591,312; 6,183,714; 6,221,330; 6,232,706; 6,303,094; 6,333,016; 6,346,189; and 6,413,487. CVD methods represent one approach for industrial scale preparation of nanotubes.
CVD is a term used to represent heterogeneous reactions in which both solid and volatile reaction products are formed from a volatile precursor, and the solid reaction products are deposited on a substrate. CVD has become a common method for thin film growth on various solid substrates. CVD of carbon has been successful in making carbon films, fibers, carbon-carbon composites and multiwalled carbon nanotube (MWNT) materials at industrial scale. Only recently, however, has the growth of single-walled carbon nanotube (SWNTs) using CVD become possible. See, for example, Dai, H., et al., Chem. Phys. Lett. (1996), 260, 471-475. Currently, both SWNTs and MWNTs can be synthesized using CVD methods.
There has been active research and product development using these nanotubes as electron source. For example, carbon nanotubes have been described for use as field emission electron sources. Other applications for nanotubes have been proposed, such as flat panel displays, x-ray devices, and so forth. For device applications, it is preferably that the electron field emission cathodes have long operating lifetime (>100 hours) and emission stability.
Electron field emission properties of nanotubes are found to depend on the structure and morphology of the carbon nanotubes. Because of the field enhancement factor, smaller diameter nanotubes tend to give a lower threshold field for emission. Experimental results have shown SWNTs tend to bundle together and that SWNT bundles tend to have a lower threshold field for emission than the MWNTs which have a larger diameter. Materials comprising individual SWNTs are expected to have an even lower threshold field than those of the SWNT bundles. However at present, macroscopic quantities of materials with discreet individual SWNTs are difficult to obtain.
Emission stability of nanotubes, especially at high emission current and current densities, depends on the quality of the nanotubes, such as the concentration of structural defects. SWNTs formed by the laser ablation method tend to have a higher degree of structural perfection than MWNTs formed by CVD methods. Experimentally, it has been demonstrated that SWNTs formed by the laser ablation methods are more stable at high emission currents than MWNTs formed by the CVD methods. The laser ablation method, however, is costly and produces a small quantity of materials.
SWNTs with a single graphene shell per tube are generally not chemically inert. They can be oxidized at elevated temperatures (>400° C.) and readily absorb chemical species on their surfaces, which can lead to changes in their electronic properties and, consequently, electron field emission properties. SWNTs can also be damaged by ion sputtering during emission leading to catastrophic failures. In the case of MWNTs with several concentric graphene shells, the inner graphene shells are protected by the outer graphene shells and, therefore, can be more chemically stable than the SWNTs.
It is therefore desirable to design and fabricate a structure that can overcome the shortcomings of both the SWNTs and regular MWNTs with large diameters for electron field emission applications.